The present invention relates to a torque sensor used for detecting a steering torque in a power steering apparatus for exerting a steering assisting force in accordance with, for example, the steering torque, and a method of manufacturing the torque sensor.
According to a torque sensor 101 of a related art shown in FIG. 10, a first shaft 102 made of a magnetic material is connected with a second shaft 103 elastically rotatably relative to each other. An outer periphery of the first shaft 102 is covered by a magnetism leakage preventing member 104 made of a nonmagnetic material and an outer periphery of the magnetism leakage preventing member 104 is covered by a first detecting cylinder 105 made of a magnetic material. The second shaft 103 is integrated with a second detecting cylinder 106 made of a magnetic material to rotate along therewith. The first shaft 102 is integrated with a third detecting cylinder 107 made of a magnetic material to rotate along therewith. One end of the first detecting cylinder 105 and one end of the second detecting cylinder 106 are opposed to each other with a clearance therebetween and other end of the first detecting cylinder 105 and the third detecting cylinder 107 are opposed to each other with a clearance therebetween. A first coil 108 constituting a first magnetic circuit by generating a magnetic flux passing the one end of the first detecting cylinder 105 and the one end of the second detecting cylinder 106 and a second coil 109 constituting a second magnetic circuit by generating a magnetic flux passing the other end of the first detecting cylinder 105 and one end of the third detecting cylinder 107 are held by a housing 110. The one end of the first detecting cylinder 105, the one end of the second detecting cylinder 106 and the one end of the third detecting cylinder 107 are constituted by pluralities of teeth aligned along a circumferential direction and the other end of the first detecting cylinder 105 is constituted by a flat face. Thereby, a reluctance with respect to the passing magnetic flux in the first magnetic circuit is changed in accordance with an elastic relative rotational amount between the two shafts 102 and 103 and a magnetic resistance with respect to the passing magnetic flux in the second magnetic circuit is made to remain unchanged in accordance with the elastic relative rotational amount between the two shafts 102 and 103. The change in the reluctance with respect to the passing magnetic flux in the first magnetic circuit is changed in accordance with the elastic relative rotational amount between the two shafts 102 and 103 and therefore, the change corresponds to a torque transmitted by the two shafts 102 and 103. The change in the reluctance with respect to the passing magnetic flux in the second magnetic circuit corresponds to a variation in temperature. Thereby, a variation in temperature of a detected torque can be compensated for by calculating the torque transmitted by the two shafts 102 and 103 based on a deviation between a value in correspondence with a change in the reluctance with respect to the passing magnetic flux in the first magnetic circuit and a value in correspondence with a change in the reluctance with respect to the passing magnetic flux in the second magnetic circuit.
In the related art, according to the torque sensor 101 of a type of preventing detection accuracy from being deteriorated by preventing magnetism leakage from the first detecting cylinder 105 to the first shaft 102 by the magnetism leakage preventing member 104 as described above, the magnetism leakage preventing member 104 is fitted together with the first shaft 102, the first detecting cylinder 105 is fitted together with the magnetism leakage preventing member 104 and the first detecting cylinder 105 and the magnetism leakage preventing member 104 are integrated to the first shaft 102 by a pin 120 (refer to JP-UM-A-3-48740).
According to the above-described torque sensor 101 of the related art, finishing of an inner periphery of the first detecting cylinder 105, working of forming pin holes to the first detecting cylinder 105, the magnetism leakage preventing member 104 and the first shaft 102 and press-fitting of the pin 120 into the pin holes are needed to thereby cause to increase working cost and integrating cost.
Further, in order to restrain a variation in temperature of the detected torque, it is necessary to control the clearance between the one end of the first detecting cylinder 105 and the one end of the second detecting cylinder 106 and the clearance between the other end of the first detecting cylinder 105 and the one end of the third detecting cylinder 107 with high accuracy. Therefore, working cost and integrating cost of the first detecting cylinder 105 and the magnetism leakage preventing member 104 are caused to increase.